1. Field of the Invention
The present invention relates to a distributed optical amplifying apparatus which can serve both as an optical transmission line and an optical amplifying medium, and more particularly, to a distributed optical amplifying apparatus which can compensate transmission loss, prevent a nonlinear optical effect, and improve an optical signal-to-noise ratio. Moreover, the present invention relates to an optical fiber cable suitable for the distributed optical amplifying apparatus, an optical communication station including the distributed optical amplifying apparatus, and an optical communication system including the distributed optical amplifying apparatus.
2. Description of the Related Art
Optical communication systems will be used in future multimedia networks, as advances in optical communication technology should enable the high bandwidth, high capacity, ultra long distance transmission required by such future multimedia networks. Wavelength division multiplexing (hereinafter abbreviated to ‘WDM’) is a significant optical communication technology being developed for this purpose, as WDM effectively utilizes the broadband characteristics and large capacity of an optical fiber.
More specifically, in WDM optical communication systems, a plurality of optical signals at different wavelengths are multiplexed together into a WDM optical signal. This WDM optical signal is then transmitted through a single optical fiber as an optical transmission line. A WDM optical communication system can provide extremely high bandwidth, high capacity, long distance transmission.
In a long distance optical communication system, since a WDM optical signal attenuates while being transmitted through an optical transmission line, the WDM optical signal must be amplified after being transmitted a certain distance. For this reason, optical amplifying apparatuses for amplifying the WDM optical signal are currently in use, and further research and development of such apparatuses is occurring.
Therefore, in a conventional WDM optical communication system, an optical transmitting station uses wavelength division multiplexing to multiplex together a plurality of optical signals at different wavelengths into a WDM optical signal. The WDM optical signal is then transmitted through an optical transmission line. An optical receiving station receives the transmitted WDM optical signal from the optical transmission line. One or more optical repeater stations are positioned along the optical transmission line to amplify the WDM optical signal. The number of optical repeater stations is typically determined in accordance with system design parameters to provide a sufficient amount of amplification.
While being transmitted through the optical transmission line, the WDM optical signal deteriorates in its waveform due to wavelength dispersion, transmission loss, and a nonlinear optical effect Therefore, various countermeasures have been devised.
For example, various conventional methods have been devised for providing wavelength dispersion compensation. In one such method, a dispersion-managed fiber (hereinafter abbreviated to ‘DMF’) combines optical fibers with different wavelength dispersion from each other.
FIGS. 1A and 1B are diagrams showing the structures of conventional dispersion-managed fibers.
More specifically, FIG. 1A shows a partial structure between two stations in an optical communication system, where an optical repeater station 1004-A and an optical repeater station 1004-B are connected by an optical transmission line 1002. The optical transmission line 1002 is composed of an optical transmission line 1002-L1 whose wavelength-dispersion is positive and an optical transmission line 1002-L2 whose wavelength dispersion is negative. An optical signal is transmitted to the optical repeater station 1004-B from the optical repeater station 1004-A via the optical transmission line 1002-L1 and the optical transmission line 1002-L2. While being transmitted, the optical signal undergoes a positive wavelength dispersion in the optical transmission line 1002-L1 and undergoes a negative wavelength dispersion in the optical transmission line 1002-L2 to be compensated in a manner that accumulated wavelength dispersion becomes almost zero. The DMF as described above is disclosed, for example, in U.S. Pat. No. 5,191,631, and Japanese Patent Laid-open No. Hei 9-318824. A symmetrical DMF in which the wavelength dispersion is made symmetrical is also disclosed.
FIG. 1B shows a partial structure of two stations in an optical communication system. An optical repeater station 1004-C and an optical repeater station 1004-D are connected by the optical transmission line 1002. The optical transmission line 1002 is composed of an optical transmission line 1002-L3 whose wavelength dispersion is positive, an optical transmission line 1002-L4 whose wavelength dispersion is negative, and an optical transmission line 1002-L5 whose wavelength dispersion is positive. An optical signal which is sent out from the optical repeater station 1004-C undergoes the positive wavelength dispersion in the optical transmission line 1002-L3, undergoes the negative wavelength dispersion in the optical transmission line 1002-L4, and undergoes the positive wavelength dispersion in the optical transmission line 1002-L5. Therefore, the optical signal is transmitted to the optical repeater station 1004-D in a manner so that compensation causes accumulated wavelength dispersion to become almost zero. Meanwhile, an optical signal sent out from the optical repeater station 1004-D undergoes the positive wavelength dispersion in the optical transmission line 1002-L5, undergoes the negative wavelength dispersion in the optical transmission line 1002-L4, and undergoes the positive wavelength dispersion in the optical transmission line 1002-L3. Therefore, the optical signal is transmitted to the optical repeater station 1004-C in a manner so that compensation causes accumulated wavelength dispersion to become almost zero. Such a DMF is disclosed, for example, in U.S. Pat. No. 5,778,128, a paper, “Enhanced power solitons in optical fibers with periodic dispersion management ” (N. J. Smith, F. M. Knox, N. J. Doran, K. J. Blow and I. Bennion: Electronics Letters, Vol. 31, No. 1, p54-p55, 4 Jan. 1996), a paper, “Energy-scaling characteristics of solitons in strongly dispersion-managed fibers” (N. J. Smith, N. J. Doran, F. M. Knox and W. Forysak Optics Letters, Vol. 21, No. 24, p1981-p1983, 15 Dec. 1966), and a paper, “40 Gbit/s×16 WDM transmission over 2000 km using dispersion managed low-nonlinear fiber span” (Itsuro Morita, Keiji Tanaka, Noboru Edagawa and Masatoshi Suzuki: ECOC 2000, Vol. 4, p25-p26, 2000).
These conventional technologies are devised from the viewpoint of wavelength dispersion compensation. Such technologies were not devised in consideration of a system in which an optical transmission line also serves as an optical amplifying medium for distributed optical amplification.
Meanwhile, various methods for compensating the transmission loss have also been conventionally devised, and a distributed optical amplifying apparatus, especially a distributed Raman amplifier, is one of them.
FIGS. 2A and 2B are diagrams showing the structures of conventional loss compensated/distributed Raman amplifiers.
FIG. 2A shows a partial structure between two stations in the optical communication system described above, where the optical repeater station 1004-A and an optical repeater station 1004-E are connected with the optical transmission line 1002. In the optical repeater station 1004-E, a pump light source 1005-E for supplying pump right used for Raman amplification is provided. The optical transmission line 1002 is composed of an optical transmission line 1002-L6 which has a large effective cross section and an optical transmission line 1002-L7 which has a small effective cross section compared with that of the optical transmission line 1002-L6, and it is supplied with the pump light from the pump light source 1005-E. An optical signal is transmitted from the optical repeater station 1004-A to the optical repeater station 1004-E via the optical transmission line 1002-L6 and the optical transmission line 1007-L7, and is Raman-amplified by the pump fight in the optical transmission line 1002 while being transmitted to be compensated in such a manner that transmission loss becomes almost zero. In other words, the optical signal is Raman-amplified so that an output optical level of the optical repeater station 1004-A and an input optical level of the optical repeater station 1004-E are substantially equal to each other. The effective cross section is a part of a cross section of the optical transmission line in which the optical signal and the pump-light interact with each other to cause sufficient Raman amplification. Such a DMF is disclosed, for example, in a paper, “40 Gbit/s×8 NZR WDM transmission experiment over 80 km×5-span using distributed Raman amplification in RDF” (R. Ohhira, Y. Yano, A. Noda, Y. Suzulki, C. Kurioka, M. Tachigori, S. Moribayashi, K. Fukuchi, T. Ono and T. Suzaki: ECOC '99, 26-30, p176-p177, September 1999, Nice, France).
Here, a size of the effective cross section correlates with a scale of the nonlinear optical effect When the effective cross section is large, the nonlinear optical effect is small. On the other hand, when the effective cross section is small, the nonlinear optical effect is large. Therefore, from the viewpoint of a choice of whether optical power in the optical repeater station 1004-A from which the optical signal is sent out is increased or optical power in the optical repeater station 1004E from which the pump light is supplied is increased, a structure as shown in FIG. 2B in also possible. In FIG. 2B, the optical transmission line 1002 is composed of an optical transmission line 1002-L8 which has a small effective cross section and an optical transmission line 1002-L9 which has a large effective cross section compared with the optical transmission line 1002-L8. The optical transmission line 1002-L8 is connected to the optical repeater station 1004-A Such a structure is disclosed, for example, in a paper, “A proposal of a transmission line without any loss in a longitudinal direction utilizing distributed Raman amplification” (Toshiaki Okuno, Tetsufumi Tsuzaki and Masayuki Nishimura: B-10-116, the 2000 Society Conference of the Institute of Electronics, Information and Communication Engineers).
These conventional technologies as shown in FIGS. 2A and 2B are technologies which are devised from the viewpoint of compensating the transmission loss and no consideration is made for the wavelength dispersion compensation, an optical signal-to-noise ratio (hereinafter abbreviated to ‘optical SNR’), and so on.
Furthermore, in the conventional arts as shown in FIGS. 1A and 1B and FIGS. 2A and 2B, the nonlinear optical effect, especially a nonlinear phase shift, is not taken into consideration.
It is noteworthy that the wavelength dispersion and the effective cross section have such a correlation that an optical fiber with the positive wavelength dispersion usually has a small effective cross section and an optical fiber with the negative wavelength dispersion usually has a large effective cross section.
In realizing long distance transmission of an optical signal with less error ratio, there is a problem that the wavelength dispersion, the transmission loss, and the nonlinear optical effect need to be compensated in a well-balanced manner as a whole instead of compensating only one physical quantity.